A. Field of the Invention
This invention relates to pulmonary function filters.
B. Related Art
Pulmonary function filters, traditionally used on medical devices such as spirometers and other respiratory function analyzers and treatment equipment, are intended primarily to remove viral and bacterial organisms from an air or other gas stream. They also remove particulates and any other matter such as moisture droplets contained in the gas stream. The size and effectiveness of the filter in removing these elements from the gas stream depend on the characteristics of the filter element used in the filter.
In the case of spirometry, the filter removes bacterial and viral organisms as well as particulates found in the exhalation stream coming from the patient. The filter also removes any matter entrained in the inhalation stream coming from the spirometer during inhalation.
The effectiveness of the filter depends on the actual filter element housed in the filter which captures the organisms and particulates. Filter effectiveness in turn depends upon proper positioning of the filter element and ensuring that the configuration of the filter element remains unchanged during normal use. If the filter element collapses, gases can pass around the edges of the filter element, and, to that extent, are not filtered. If a filter element contacts the wall of the filter housing, little if any gas flow passes from one side of the filter to the other at the area of contact, thereby reducing the effective surface area of the filter element. Gas flow may thereby be reduced, increasing the back pressure inhibiting free flow of gas through the filter.
Fins or pins may be used on the surface of the filter housing to hold the filter element off the housing wall. Fins form eddy currents and vortices in the gases flowing through the filter, and somewhat reduce the free gas volume within the filter housing. They also form condensation points for moisture in the gases passing through the filter. Consequently, they reduce filter effectiveness. If the circumferential edges of the filter element are clamped between circumferential edges of two halves of the filter housing, the filter element might be held in place approximately centered in the filter housing in order to hold the filter element taut. However, the filter element can flex and loosen from between the two halves of the filter housing reducing the overall effectiveness of the filter.